Although water is one of the most abundant resources in nature, the availability of fresh water is a problem in many regions of the world. Increasing demand and changing climate may worsen the fresh water availability problem. In developing countries, many diseases and deaths are attributed to the poor quality of water. The scarcity of fresh water is expected to become more serious in those countries, and also in the Middle East and North African countries that are suffering from lack of renewable fresh water. The number of people who will face water shortage in the year 2025 is estimated about four times larger than the number of those in the year 2000. This water shortage problem has necessitated a variety of desalination technologies and methods for brackish water and sea water. Some of the representative approaches are thermal distillation and membrane techniques such as multi-stage flash distillation and reverse osmosis. While these methods have been used and improved upon for a number of years, they still have many deficiencies that can be improved. These deficiencies include the relatively high energy requirements, high cost, fouling issues, pre-treatment requirements, and the low flux which many of the current, membrane-based devices and methods experience.
Multi-stage flash distillation is the representative thermal approach for desalination and is utilized mainly in Middle East countries. This method basically comprises a series of evaporation and condensation steps. The feed salt water is heated by steam and then a portion of the heated feed water is vaporized by being exposed to a lower pressure stage. The remaining water is introduced to the next stage where pressure that is lower than pressure in the previous stage is applied to the water. The process continues until sufficient water is processed to recover distilled water by condensation. To increase the thermal efficiency, it is common that the condensation occurs on the contact to pipes transporting cold feed water, and this condensation process has a role of pre-heating for the feed water. The main disadvantage of this approach, however, comes from the high energy requirement due to the high latent heat of water, which can be approximately 290 kJ/kg to desalinate sea water in some instances.
While membrane-based reverse osmosis is more economical than the thermal processes with respect to the cost of energy, it too involves its own challenges. For instance, a large fraction of the cost for desalination arises from membranes and their maintenance. Further, although increasing the membrane flux may decrease the required membrane area and capital costs, it requires large pressures that lead to viscous losses and decreased efficiency. The cost of producing current membrane-based reverse osmosis systems increases as the concentration of the solute increases. Still further, reverse osmosis membranes suffer from fouling, scaling, viscous losses, having weak resistance to chlorine, having insufficient boron rejection, and having low mass flux per unit area of the membrane, which in turns requires large membrane areas. Further improvement in reverse osmosis hinges on improvement in flux and fouling resistance, which is determined primarily by membrane structure and composition.
Hybrid distillation methods such as direct contact membrane distillation, which uses a hydrophobic membrane in between salt water and purified water, rely on temperature differences to evaporate and condense water. Direct contact membrane distillation is highly inefficient as there is a huge heat loss across the membrane by conduction, and other distillation approaches have to deal with heat transport from the condensate to the evaporating salt water. Other variations of membrane distillation, such as air-gap membrane distillation and vacuum-enhanced membrane distillation, also suffer from similar drawbacks.
Accordingly, there is a need for liquid filtration methods, particularly for desalinating water and purifying water, that are energy efficient, avoid fouling and viscous losses, support high flux, and do not rely on temperature differences in order to filter the liquid.